This invention relates to the coating of air bags, which are used for safety purposes to protect occupants of vehicles such as automobiles, and of air bag fabrics intended to be made into air bags. In particular the invention relates to the top coating with a friction reducing composition of air bags and air bag fabrics which have been coated with a cured organopolysiloxane composition. The invention also relates to coating compositions for overcoating air bags and air bag fabrics which have been coated with a cured organopolysiloxane composition.
Air bags are generally formed from a woven or knitted fabric made of synthetic fibre, for example of polyamide such as nylon-6,6 or polyester, covered on at least one of its sides with a layer of an elastomer. Air bags may be made of flat fabric pieces which are coated and then sewn together to provide sufficient mechanical strength, or may be woven in one piece with integrally woven seams. Sewn air bags are generally assembled with the coated fabric surface at the inside of the air bag. One piece woven air bags are coated on the outside of the air bag. A preferred elastomer for coating the air bag or air bag fabric is a silicone elastomer which is a cured organopolysiloxane composition, particularly a silicone rubber coating cured by hydrosilylation, that is by the reaction of alkenyl groups of one polyorganosiloxane and Si—H groups of another polyorganosiloxane.
Air bags coated with a silicone elastomer are described in many published patents and applications, for example U.S. Pat. Nos. 5,789,084, 5,877,256, 6,709,752, 6,425,600 and 6,511,754, and WO-A-08/020605 and WO-A-08/020635.
For some airbag applications, pressurised gas has to be retained in a fabric envelope for a relatively long period. This requirement exists for example in side curtain airbags for the automotive industry. These side curtain airbags are intended to inflate at the time of impact, as do conventional airbags. The side curtains unfold to form a cushioned curtain between passengers and some of the side of the car body, e.g., the windows. As the intention is not merely to cushion the blow on impact itself, as is the case for conventional driver and passenger airbags, but to protect passengers e.g. when a car is rolling, it is important that the side curtain air bag is sufficiently pressurised during such rolling process. Where conventional driver and passenger airbags only need to retain pressure for a fraction of a second, it is desirable that side curtain airbags maintain a suitable pressure for a few seconds. Similar applications exist where a pressurised fabric structure is desired to maintain a certain gas pressure for a relatively extended period of time, e.g. in emergency chutes for aeroplanes, or inflatable rafts. There is thus a demand for coated fabrics having the benefits of flexibility and high temperature resistance at low coating weight given by silicone rubber coatings, but with improved air tightness.
Silicone elastomer coatings on air bags have an excellent combination of flexibility and low enough gas permeability of the coated fabric. However, silicone elastomer coatings have a high surface friction when cured. If a silicone elastomer base coat is left as the only coating on the air bag, the surface properties of this base coat would result in blocking (the silicone surfaces sticking to each other during storage and tight packing of the air bag in the automobile) and very high stresses when the airbag is inflated which would result in bag failure by tearing during inflation.
U.S. Pat. No. 6,177,366 describes airbag coating compositions comprising at least two separate and distinct layers. The first layer (base coat), being in contact with the airbag surface, comprises a silicone elastomer. The second layer (topcoat) is preferably a silicone resin.
U.S. Pat. No. 7,198,854 describes an anti-friction silicone varnish for textiles coated with silicone elastomers. The varnish comprises a crosslinkable silicone composition containing two silicones which react with one another in the presence of a catalyst to allow crosslinking, and a particulate component comprising powdered (co)polyamides.
We have found that crosslinked silicone compositions do not have a sufficiently low coefficient of friction to avoid blocking of the air bag surfaces during storage and tearing of the air bag during inflation unless highly filled with a low friction filler such as talc. Air bag manufacturers do not like such highly filled compositions as the talc settles during storage and the composition is difficult to apply consistently at low coat weights.